The following disclosure relates to electrical circuits and signal processing.
A signal in a transmit or receive path of a communications transceiver can contain undesired spurious tones, and the spurious tones can degrade the quality of the signal. Spurious tones can be caused, for example, by a local-oscillator signal or a signal at a mixer input coupling to a mixer output. Spurious tones can also be caused by clock harmonics in a local-oscillator signal or noise from other parts of the communications transceiver capacitively coupling to a signal conduit. A received signal can contain spurious tones from a remote transmitter. A communications transceiver can filter signals in the transmit or receive path to attenuate spurious tones. Some conventional transceivers use external filters to attenuate spurious tones in a transmitted or received signal. Integrated communications transceivers can use on-chip filters instead of external filters to save space and to lower component costs.
Conventional on-chip filters in a transceiver can include a serial or parallel connection of a capacitor and an inductor, hereafter referred to as a serial or parallel LC circuit, respectively. FIG. 1A shows a graph 110 of the magnitude of an impedance of parallel LC circuit 120 versus frequency. The impedance of circuit 120 shown in graph 110 is present between terminals 135 and 140. Graph 110 has a peak 115 where a capacitor 125 and an inductor 130 resonate. The frequency at which peak 115 is located corresponds to a pole in a transfer function of circuit 120. When an input current is passed through terminals 135 and 140 and an output voltage is measured between terminals 135 and 140, tones at or near the frequency at which peak 115 is located are passed with less attenuation than tones at frequencies other than the frequency at which peak 115 is located. When inductor 130 and capacitor 125 are ideal components, the impedance of peak 115 is infinite. Because inductor 130 and capacitor 125 typically include parasitic resistance, the impedance of peak 115 is typically finite.
FIG. 1B shows a graph 150 of the magnitude of an impedance of serial LC circuit 160 versus frequency. The impedance of circuit 160 shown in graph 150 is present between terminals 175 and 180. Graph 150 has a dip 155 where a capacitor 165 and an inductor 170 resonate. The frequency at which dip 155 is located corresponds to a zero in a transfer function of circuit 160. When an input current is passed through terminals 175 and 180 and an output voltage is measured between terminals 175 and 180, tones at or near the frequency of dip 155 are attenuated.
Impedance in a conventional LC circuit can be tuned to attenuate a spurious tone (e.g., a tone at the frequency of dip 155 in FIG. 1B). However, when a transmitted signal in a wireless transmitter contains spurious tones, a single LC circuit may not attenuate spurious tones adequately (for example, to satisfy Federal Communications Commission regulations), so multiple LC circuits can be cascaded. Cascaded LC circuits that adequately attenuate spurious tones can also attenuate a desired signal significantly. If a spurious tone is close in frequency to the desired signal, conventional LC circuits may not be able to adequately attenuate the spurious tone while preserving the desired signal.